Conventionally, automobile navigation systems mounted in vehicles set a route to a destination and provides a driver with guidance (route guidance) using graphics and voice according to the set route.
For example, audio guidance related to a course at an intersection is provided by taking the following procedure: the distance from the relevant vehicle to the intersection is computed based on (i) the absolute position computed from observational data from GPS and the like and (ii) map data used in display of the present position and set routes and the like; the distance to the intersection is reported by voice, together with a course at the intersection.
In general, such audio guidance is singly provided at some point in time when the vehicle has approached an intersection. Therefore, when the driver is paying attention to the behavior of other cars, for example, the driver can miss the guidance. Various measures are known to cope with this. For example, a driver's reaction to audio guidance is detected through the presence or absence of a turn-signal signal. When the driver's reaction is not in accordance with the instruction of audio guidance, audio guidance is provided again. (Refer to Patent Document 1, for example.)                Patent Document 1: JP-H9-329456 A        
However, these measures involve a problem. The timing with which a driver lights up a turn signal when turning to the right or left at an intersection differs from driver to driver. Therefore, though a driver attempts to drive his/her car in accordance with a route set by an automobile navigation system, audio guidance can be repeated. This causes driver discomfort.